Yes, the labeling of carbs and other macronutrients per 100g is a requirement in many countries around the world, but sadly not in the USA, and indeed manufacturers can choose whatever serving size they want in order to manipulate the numbers into making people believe their products are healthier than they are. A total disgrace if you ask me that such nonsense is allowed, but that's the way it is.

Heavy whipping cream / whipping cream definitely has some carbs, and for some of us doing extremely low carbs, trying to consume a lot of it can use up most of our daily carb allotment. I know, because at times I was doing a maximum of 10g of carbs per day, and easily could consume 5g or more in just heavy whipping cream. If you want to really know how many carbs are in a food, use one of the Australian or other countries' websites that lists nutritional information, and they'll likely have the ability to see how many carbs per 100g, then calculate it out based on how much you're using.

I learned this the hard way when I moved to Germany from the US a few weeks ago. In Germany they put all carbs on the label, even if, as in the case of butter for example, it is 0.4 g of carbs. My can of sardines says 0.7 carbs. In the US, heavy cream claims to have 0 carbs whereas here in Germany, the same type of cream without any sugar added, has 3.3g of carbs. I think the German labels are more accurate.